The ruling—tied to gains that Mr. Thaksin's family made from selling a stake in Shin Corp. PCL—could anger Mr. Thaksin's supporters and ignite a fresh wave of political upheaval.

According to the verdict from the Supreme Court's Criminal Division for Holders of Political Positions, 46.37 billion baht of Mr. Thaksin's frozen assets, plus interest, will be seized.

The judges said they decided on the amount because it was equal to the difference in value of Shin Corp. shares from the date before he came to office and the value when the shares were sold to Singapore's Temasek Holdings in early 2006.

The court ruled the fugitive ex-leader was found to be the actual owner of Shin Corp. shares while in power, which violated the law. Mr. Thaksin also was found guilty of abuse of power for using his authority as the government leader to grant privileges to Shin Corp.'s units.

Ahead of the verdict, Thai shares closed 0.6% higher at 721.37, while the dollar edged down to 33.04 to 33.05 baht from 33.06 to 33.065 baht late Thursday as there were no signs of violence.

Mr. Thaksin was the founder of Shin Corp., and his family earned 76 billion baht tax-free when it sold its stake to Temasek, infuriating his opponents. The family wasn't obligated to sell because of Mr. Thaksin's role as prime minister, a post he had held since 2001, but decided to do so.

The tax-free nature of the deal triggered mass protests and eventually led to a bloodless military coup in September of that year.

Mr. Thaksin's supporters are mostly poor farmers in northern and northeastern provinces, and low-income earners in urban areas. Identified by their red shirts, Thaksin supporters have staged a number of antigovernment rallies over the past year, including riots last April during a summit of the Association of South East Asia Nations. ASEAN leaders fled the venue, humiliating the host Thai government.

Thousands of security forces were deployed across Thailand Friday ahead of the verdict in preparation for any possible violence. Security was also tight at the Supreme Court, but Bangkok was largely operating as normal.

Mr. Thaksin fled Thailand in late 2008 after a court convicted him of abuse of power and sentenced him to two years in prison. He continues to generate enormous divisions in Thailand, often taunting Prime Minister Abhisit Vejjajiva from Dubai—where Mr. Thaksin now is based—and addressing supporters by telephone or video-link.